Park Tools

Pacific Decadal Oscillation

Many things affect the Northwest's weather. One is the Pacific Decadal Oscillation (PDO). While an El Niño or La Niña typically persists less than 18 months, the PDO lasts 20 to 70 years.

Locations and shifts of cold and warm water in the Pacific alter the path of the jet stream. Changes in the jet stream result in both short and long-term changes in weather patterns.

Research has suggested that temperature and pressure changes in the North Pacific and the resultant impact on the Northwest climate could be explained simply by a "shift" in PDO phase. Like El Niño or La Niña, the PDO comes in two distinct conditions. During the warm (positive) PDO phase, the waters in the central North Pacific are cool, and those along west coast of North America are warm. The opposite is true with the cool (negative) phase.

While the PDO is a longer-term natural oscillation when, its effects are combined with El Niño or La Niña conditions, temperatures and precipitation amounts are affected, particularly during the winter. For instance, the warm or positive phase of the PDO can enhance an El Niño episode, amplifying the effects of this warm pool of water on the western U.S. This pattern predominated from the mid-1970s through at least the late 1990s and was a period of more frequent and stronger editions of El Niño. Similarly, during a negative phase of the PDO, La Niña conditions are more likely to be enhanced. Currently we are in a negative phase of the PDO.

Recently, unusual atmospheric conditions, not consistent with the PDO or other phenomena, have been observed in the North Pacific, leading some scientists to believe the ocean's part in influencing the weather is still underestimated.

Beyond directly impacting the weather of the Northwest, evidence also suggests that marine ecosystems are influenced by changes in climate. Many questions about these influences remain. In past years, salmon populations have increased off the Pacific Northwest and in Alaska. Under classic PDO scenarios, including during its distinct negative phase from 1947 to 1976, Alaska's stocks would become depleted. This inconsistency may offer more evidence that researchers are still missing several pieces of the climate puzzle.